A hard disk drive is known as a type of information storage device. A hard disk drive typically includes one or more magnetic disks rotatably mounted in association with a spindle and one or more actuator assemblies for positioning a magnetic transducer, or head, relative to concentric circular data tracks on a magnetic medium-bearing surface of each disk.
The recording densities of hard disk drives have been increasing with advances in personal computers, so that data tracks are becoming increasingly more densely positioned on the disks, and the tracks themselves are becoming physically narrower. As a result, maintaining the transducer or head in an accurate track-following position for purposes of reading and writing, is becoming more difficult. To accommodate the needed increasingly finer adjustments in the position of the magnetic head, a precision positioning actuator has been introduced. For such an actuator, a single piezoelectric actuator assembly is generally employed. The piezoelectric actuator forms a part of a head gimbal assembly which moves the head transverse to a track to provide fine radial positioning of a head in reference to a track.
In such configurations in the prior art, each piezoelectric actuator typically includes a piezoelectric crystal extending along a drive axis, and disposed between parallel electrically conductive plates extending transverse to the drive axis. Each piezoelectric actuator is responsive to an applied voltage control signal applied across its plates to effect a desired physical displacement of one of its plates with respect to the other. One difficulty that arises in the use of piezoelectric actuators is a hysteresis effect. Due to the hysteresis effect, a curve that describes the displacement response of a piezoelectric element to applied voltage, tends to form a loop rather than straight line. As a result, the relative displacement of its plates is not precisely linear with voltage, and, moreover, is not a single-valued function of the applied voltage.
In addition, because of the hysteresis, the steepness of the displacement dependence on the control voltage varies when the control voltage is changed. As a result, when deployed in a control feedback loop, the loop gain depends on control voltage changes, with a consequent decline of the bandwidth of the feedback loop. That decline is another reason for deterioration of the write/read head positioning precision in the prior art.
The problem of hysteresis may be overcome if the displacement of a piezoelectric actuator is controlled through an electric input control signal by applying to the actuator, an electric charge proportional to the input signal, rather than by applying a corresponding voltage or current. Such a way to improve the piezoelectric actuator performance is possible since the hysteresis loop of a charge-deformation characteristic curve is generally much smaller than that of a voltage-deformation characteristic curve for a piezoelectric crystal structure.
A prior art driving circuit that provides control of a piezoelectric actuator in a charge mode, was proposed in U.S. Pat. No. 4,263,527, entitled “Charge Control of Piezoelectric Actuators to Reduce Hysteresis Effects”, issued to Robert H. Comstock. A block diagram of the circuit disclosed in that patent is shown in FIG. 1. The block diagram comprises a piezoelectric actuator 13 having a first terminal 13A and a second terminal 13B, a fixed value capacitor 15, having electrodes 15A and 15B connected in series between the second terminal 13B of actuator 13 and ground, an operational amplifier 11 having a non-inverting input terminal 10, an inverting input terminal 12, and an output terminal 11A, where the non-inverting input terminal 10 is adapted to receive an input control signal, and where the output terminal 11A of operational amplifier 11 is coupled to the first terminal 13A of actuator 13. A buffer amplifier 14 includes an input terminal 14A coupled to the second terminal 13B of actuator 13 and an output terminal 14B coupled to the inverting input terminal 12 of operational amplifier 11. Upon application of an input control signal to non-inverting input terminal 10 of operational amplifier 11, the voltage at output terminal 11A of amplifier 11 induces opposite charges on opposite surfaces of a piezoelectric element incorporated in the piezoelectric actuator 13, where those opposite surfaces are coupled to a respective one of the first terminal 13A and second terminal 13B of actuator 13. In the series connection of the piezoelectric actuator 13 and capacitor 15, since buffer amplifier 14 is characterized by high input impedance, the same current (corresponding to the charge transfer occurring at the surfaces of the piezoelectric element of actuator 13) flows through both elements. For this reason, the current induces an equal charge differential between the electrodes 15A and 15B of the capacitor 15, thereby producing a voltage proportional to the charge on the actuator 13 between the upper terminal 15A of the capacitor and the lower electrode 15B, coupled to ground. This voltage is fed back through buffer amplifier 14 to the inverting terminal 12 of operational amplifier 11. When the input voltage changes, the output of the amplifier 11 is changed, and the charge on actuator 13 is changed too. The feedback arrangement thereby quickly forces the charge on the actuator 13 to a value proportional to the input control signal 10.
The impact of hysteresis on the precision of piezoelectric actuator operation is essentially eliminated in the driving circuit of the FIG. 1. However, the suggested solution suffers from some drawbacks that prevent its use as such in a hard disk drive and in head and disk test equipment.
More particularly, in most cases, one plate of the piezoelectric actuator incorporated in a head gimbal assembly is connected to a reference terminal, such as ground. For this reason, it is not possible to insert a capacitor between the piezoelectric actuator and the ground, as suggested in U.S. Pat. No. 4,253,567.
In practice, a head gimbal assembly, has only one wire that connects it with the control unit of the disk drive (or head/disk test equipment) and which may be used for control of the piezoelectric actuator-driven displacement of a head. However, the prior art driving circuit of FIG. 1 requires two couplings: one from the amplifier 11 to the piezoelectric actuator 13 and one from the capacitor 15 to the buffer (feedback) amplifier 14.
The circuit between the capacitor 15 and feedback amplifier 14 has high impedance loads at both of its ends. Such a circuit is susceptible to the noise and electromagnetic interference (EMI) from the environment, which is one of the primary sources of the interference that decreases the accuracy of the operation of a piezoelectric actuator.
An improved prior art driving circuit for controlling a piezoelectric actuator that can be used in a disk drive, was proposed in U.S. Pat. No. 6,246,152, entitled “Driver Circuit for Controlling a Piezoelectric Actuator in Charge Mode”, issued to Luca Fontanella et al.
A block diagram that illustrates the operation of the driving circuit described in the '152 patent is shown in FIG. 2 herein. In that figure, a control signal is applied to a non-inverting terminal 20 of a differential input stage 21. The output of differential input stage 21 is connected through a main amplifier 26 to a piezoelectric actuator 23, and through an auxiliary amplifier 24 to an inverting terminal 22 of differential input stage 21. The inverting terminal 22 of differential input stage 21 is coupled to ground by a capacitor 25.
Due to feedback loop operation, a voltage across capacitor 25 only slightly, or not at all, differs from the input control signal at terminal 20. Charge stored in the capacitor 25 is proportional to the voltage between its terminals, and hence it is proportional to the input control signal 20.
In the illustrated configuration of the '152 patent, the main amplifier 26 is identical to the auxiliary amplifier 24. Since the inputs of the amplifiers 26 and 24 are connected in parallel, the current produced by main amplifier 26 is equal to head gimbal assembly drive current at the output of auxiliary amplifier 24. As a result, charge stored between terminals of the piezoelectric actuator 23 equals the charge stored on capacitor 25, and is proportional to the input control signal. The fact that the piezoelectric actuator 23 in the '152 patent's configuration shown in the block diagram of FIG. 2 herein, is driven by charge instead of voltage, provides a desired suppression of hysteresis effect in actuator operative characteristics.
However, it is important to note that in the '152 patent's configuration shown in FIG. 2 herein, the charge storage capacitor 25 is not directly coupled to the piezoelectric actuator, as was the case in the configuration of FIG. 1. That allows capacitor 25 to be located in a driving circuit away from the immediate vicinity of the piezoelectric actuator 23, avoiding one of the above-noted disadvantages of the configurations of U.S. Pat. No. 4,263,527. With the driving circuit located away from piezoelectric actuator (and the read/write head), that allows a reduction in the number of wires between the driving circuit and the piezoelectric actuator 23 down to one. The piezoelectric actuator 23 in the block diagram of FIG. 2 herein is connected by one of its terminals to ground. Thereby as a result, piezoelectric actuators can be employed with one grounded terminal (as usually is the case in hard disk drives). However, there still remains a significant problem: noise immunity of the driving circuit of configurations of the prior art type of FIG. 2 herein is poor as also is the case in configurations of the prior art type of FIG. 1 herein.
In configurations of the type of FIG. 2, the amplifiers 24, 26 should produce current proportional to the input voltage; that is, those amplifiers are to operate as current sources. For this reason, the output impedance of those amplifiers is high. The circuit 27 in FIG. 2 herein connects the piezoelectric actuator 23 to the main amplifier 26 (located in a driving circuit at a distance from the piezoelectric actuator 23). This circuit is loaded at one end by high ohmic impedance of the amplifier 26 output, and at the other end, by a high ohmic impedance of the piezoelectric actuator 23. As a result, this circuit is compromised in large extent to induced noise and EMI present in the equipment. The induced noise is generally transmitted to the piezoelectric actuator 23, giving rise to random variations of produced displacements and deterioration of the read/write head positioning accuracy.